This invention relates generally to the field of built-up mirrors and more particularly to improvements in light weight mirrors suitable for use in space telescopes or other light or laser beam collecting or reflecting devices.
Orbital space vehicles, and in particular mirror devices carried thereby, are subjected to wide excursions in degree of exposure to sunlight which, if not compensated for in some manner, would result in such temperature induced distortions that the effectiveness of the device would become seriously degraded. In order to keep the weight of mirrors down it has been the practice to provide a reflective face plate that is backed by a cellular stiffening structure, wherein the cells may have any desired shape such as square or a honeycomb configuration, and usually with a back or rear plate. While various other materials have been used or proposed for such mirrors, glass of the type known as ULE (ultra low expansion) remains in use and it is principally to such mirrors that this invention has application. Typically, the glass front, back, and cell wall elements are bonded together by fusion of glass frit to make a unitary structure of high rigidity, but characterized by the relatively low thermal conductivity of ULE.
The expedients considered or used heretofore for avoiding or reducing temperature induced mirror distortion include heating means for adding heat to the back plate to reduce front to rear temperature differentials during periods when the front plate is being heated by exposure to the sun, the use of opaque front reflective coatings to reduce greenhouse effect in the cell structure, and the inclusion of circulated tempering fluids or the like. Of course, those expedients, especially the use of heaters, fluid circulation and the like are complex, expensive, and add materially to the weight of the system, all of which are important considerations in a system to be used in an orbital space vehicle.